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Full text of "Text Book Of Mechanical Engineering"

Appendix  VI.                          1169

or i % per 5 rise ; while a vacuum of 28 ins. mc-rcury saves 50 %,
shewing the importance of condensation with large quantities of
cold water. The best positions for turbines are therefore where
water is cheap. With these precautions a consumption1 of 14 Ibs.
steam per B.H.P. hour may be easily obtained, the rate being
almost constant down to loads of \ the maximum. Non-condens-
ing turbines also compare favourably with non-condensing recip-
rocating engines, but nothing so well as condensing.

forms.    The forms of steam turbines are divided, like water
turbines, into radial and parallel flow, and into

1.  Single stage (one wheel) impulse turbines.

2.  Multiple stage impulse turbines.

3.  Single-stage reaction or pressure turbines.

4.  Multiple-stage reaction turbines.

5.  Partial impulse and reaction turbines.

6.  Pelton-wheel turbines.

The third is represented by Hero> and in water by Fig. 709,
p. 723. It is extremely wasteful. No. i has been made success-
ful by de Laval> No. 2 by Rateau and others, No. 4 by Parsons,
and No. 6 by Stumpf. The distinction between impulse and
pressure wheels is not so clear when expansive fluid is used, and
all steam wheels work partly in each manner. t It is impossible to
obtain the full impulse jf- per Ib. weight of fluid, for the velocity
J# of the wheel periphery would then be too excessive, so al 1
impulse wheels work partly by pressure. Generally, if the steam
is allowed to acquire a high velocity before entering the wheel the
impulse character is accepted, and in multiple-stage impulse
turbines the steam-way should widen more rapidly than in the
pressure turbines. A gradually increased pressure is necessary in
both forms, and Parsons' and Rateads turbines, though specified
as pressure and impulse respectively, partake more or less of
both characters, the steam giving up its energy t^y impulse due to
change of velocity and by pressure due to reaction. The final
velocity regarding wheel and the final pressure are both very low.
The only purely impulse or pressure wheels are those of de
Laval or Hero respectively. In the former the pressure energy of
the reservoir steam is changed into velocity energy, the fluid meet-